My kids are 3.5 and I try to involve them in cooking as much as possible. With some dishes, there's just not that much for them to do right now (although peeling an onion or a few cloves of garlic can interest them for a while...) but with other dishes -- cornbread to go with chili, for example -- there's so much measuring and pouring and mixing...
Yes, it takes a lot longer to do it this way and there is substantially more clean-up. It might not be practical for WOHParents who need to get dinner on the table fast. But right now it's working for us!
I'm also looking for a set of child-safe knives that really cut. I think that there are some available through Pampered Chef but I'm looking for another source.
FWIW, I don't really find that my kids are more interested in eating meals they've helped prepare. I've often seen that cited as a reason to get kids involved in food prep, but it hasn't borne out for us. They do love helping, though.